Typically manual and automatic vehicle transmissions are equipped with at least one clutching mechanism (or torque transmitting device). The clutch functions to either mechanically link or disconnect two rotating shafts so that they spin at the same angular velocity or at different speeds. The clutch does so by meshing a clutch plate and clutch disc that is lined with a friction material. The more durable (or resistant to failure) the friction material lining the clutch disc is, the longer the clutch will be effective.
There are a plethora of friction materials compatible with vehicle clutch discs made by a variety of manufacturers. Each has its own fatigue life for a given temperature and compressive stress. Therefore, certain friction materials are more suitable for some applications over others. For example, a disc brake may require a friction material that is more durable under high compressive stresses while a transmission gear clutch may require a clutch disc with friction material that is more durable in heated working environments.
As the clutch plate or flywheel rotates it generates kinetic energy equal to its angular velocity, ω, squared multiplied by the clutch plate's mass moment of inertia, Im, divided by two: Wrotation=Imω2/2. To halt the clutch plate (as done in braking) a compressive energy greater than or equal to the kinetic energy generated by rotation of the clutch plate must be applied to the clutch plate by the clutch disc. The energy of a non-rotating clutch disc is equal to its actual kinetic coefficient of friction, μk, multiplied by its mass, m, and its linear velocity, v, squared, divided by two: Wlinear=μkmv2/2. When the application of the clutch disc does not immediately link and alter the rotation of the clutch plate slip occurs. This slip dissipates thermal energy characterized by a temperature change. The thermal energy resulting from slip is equal to the mass, m, of the clutch plate multiplied by its specific heat, c, and the change in temperature, ΔT: Wthermal=mcΔT.
Testing can be performed to measure the compressive strength of the friction material lining the clutch disc, indicating the number of cycles to failure. However, such tests do not account for the rotational energy generated by the clutch plate nor the heat dissipated upon slip.